The State Department said that Washington would hold Moscow directly accountable for such an escalation but did not elaborate on potential consequences. Department spokeswoman Jen Psaki said, however, that she could not confirm if Russia had in fact made such threats.

Earlier Monday, a Ukrainian military spokesman said Russia had issued an ultimatum to the crews of two Ukrainian warships in Crimea, demanding that they immediately surrender or be stormed and seized.

Vladimir Anikin, a Russian defense ministry spokesman in Moscow, dismissed the report of a Russian ultimatum as nonsense, but refused to elaborate.

Secretary of State John Kerry is leaving for Ukraine late Monday and then will travel to France and Italy. He had planned to see his Russian counterpart in Paris, but Psaki said that meeting was no longer certain.

The U.S. and its allies are weighing sanctions on Moscow, in what amounts to a sudden reprise of Cold War sensibilities. Once consideration is whether to bolster defenses in Europe in response to Russia's military advances on Ukraine. Kerry said Sunday that world leaders "are prepared to go to the hilt in order to isolate Russia with respect to this invasion."

Much as when superpower tensions ruled world affairs, missile defense systems and troop levels in Europe have again become urgent questions in Washington and beyond, a renewed reality that may force President Barack Obama's administration to give up its intended foreign policy shift to Asia indefinitely.

Also echoing the era of East-West confrontation, there appears to be little if any taste in the West for a direct military response to Russia's provocation.

Russian President Vladimir Putin gave no indication that he would heed the West's warnings. Hundreds of armed men surrounded a Ukrainian military base in Crimea, a pro-Russian area. In Kiev, Ukraine's capital, Prime Minister Arseniy Yatsenyuk alerted allies that "we are on the brink of disaster."

"This is absolutely the most serious test of our alliances since the Cold War ended," Rep. Marcy Kaptur, D-Ohio, said in a nationally broadcast interview Monday.

"I think it is extremely dangerous. Ukrainians fight and Russians fight," said Kaptur, who has traveled to Ukraine on several occasions and is considered an expert on that part of the world.

Senior Obama administration officials said they believe Russia now has complete operational control over Crimea and has more than 6,000 forces in the region. The U.S. was also watching for ethnic skirmishes in other areas of eastern Ukraine, though the officials said they had not yet seen Russian military moves elsewhere. The officials were not authorized to publicly discuss the situation and spoke on condition of anonymity.

Kerry said he has consulted other world leaders and all are committed to doing what is necessary to isolate Russia diplomatically. President Barack Obama spoke Sunday with German Chancellor Angela Merkel, British Prime Minister David Cameron and Polish President Bronislaw Komorowski.

Kerry planned to travel to Kiev on Tuesday for meetings with the Ukrainian government. Treasury Secretary Jacob Lew said the United States is ready to work with other countries and the International Monetary Fund to provide support for Ukraine's economy.

In Brussels, NATO's secretary-general, Anders Fogh Rasmussen, said Russia's actions have violated a U.N. charter. He said the alliance was re-evaluating its relationship with Russia.

"There are very serious repercussions that can flow out of this," Kerry said.

Beyond economic sanctions and visa bans, freezing Russian assets, and trade and investment penalties, Kerry said Moscow risks being booted out of the powerful Group of Eight group of world powers as payback for the military incursion.

Former Ambassador Nicholas Burns said, "Putin's not going to back off. ... What can President Obama do? Be very minded in opposition. We can't follow a military policy. This has to be diplomatic."

Several U.S. senators also called for bolstered missile defense systems based in Poland and the Czech Republic.

Russia is "going to be inviting major difficulties for the long term," said Kerry. "The people of Ukraine will not sit still for this. They know how to fight."

Still, it was clear that few in the West were prepared to respond immediately to Putin with military force.

At the Vatican, Pope Francis used his traditional Sunday midday appearance in St. Peter's Square to urge world leaders to promote dialogue as a way of resolving the crisis in Ukraine.

Sen. Marco Rubio, R-Fla., discussing the potential of U.S. military strikes against Russian troops in Crimea, said, "I don't think anyone is advocating for that." One of the administration officials indicated that the U.S. was not weighing military action to counter Russia's advances, saying the Obama administration's efforts were focused on political, economic and diplomatic options.

Rubio and fellow GOP Sen. Lindsey Graham of South Carolina said the Obama administration should return to plans it abandoned in 2009 to place long-range missile interceptors and radar in Poland and the Czech Republic.

Russia believed the program was aimed at countering its own missiles and undermining its nuclear deterrent. The White House denied that and has worked instead to place medium-range interceptors in Poland and Romania -- aimed at stopping missiles from Iran and North Korea.

Experts said potential U.S. budget cuts to Army units based in Germany also could be slowed, or scrapped completely, to prevent a catastrophic erosion of stability and democracy from creeping across Europe.

The Pentagon is considering new reductions to Army units in Germany that already have been slashed under Obama. Currently, there are two Army brigades -- up to 10,000 soldiers -- based in Germany, where armored and infantry units have dug in since World War II. At the end of the Cold War, more than 200,000 American forces were stationed across Europe.

Damon Wilson, an Eastern European scholar, former diplomat and executive vice president of the Washington-based Atlantic Council think tank, said the U.S. must be ready to pour its efforts into Ukraine, even at the cost of policies and priorities elsewhere.

"We should be no longer deluded by the fact that Europe is a safe spot of stability and security, and not a security risk for the U.S.," Wilson said Sunday. He said that if Putin goes unchecked, it could result in war -- the second one on NATO's borders.

The 3-year-old civil war in Syria is already a crisis for neighboring Turkey, a NATO member state. Ukraine is not a NATO member, but it borders four nations that are -- Poland, Slovakia, Hungary and Romania.

Russia has made clear it is ready to provide weapons and military equipment to governments across the Mideast that have irked Washington. Russia's permanent seat on the U.N. Security Council gives it veto power over major world deliberations.

"The challenge is, we do need to have some kind of working relationship with Russia?" Rep. Adam Schiff, D-Calif., asked Sunday. "And while we can impose these costs and take these steps, we've got to be mindful of the fact that they can impose their own costs on us."

Kerry appeared Sunday on CBS' "Face the Nation," ABC's "This Week" and NBC's "Meet the Press." Rubio was on NBC, while Graham and Schiff were interviewed on CNN. Kaptur and Burns appeared Monday on CNN.